Also safe mode did not work - same issue

Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again; * Make sure your Internet security software is up-to-date (i.e. you are running the latest version). * Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see '''[https://support.mozilla.org/en-US/kb/configure-firewalls-so-firefox-can-access-internet Configure firewalls so that Firefox can access the Internet.]''' {web link} ------------------ If there is still a problem Firefox v58 may have problems on your system. [https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Install Older Version Of Firefox] {web link} Be sure to read everything here. If you still want to downgrade, look under; '''I still want to downgrade. ''' Click the '''Directory of other versions and languages''' link. Look for the directory of the version that you want. But, remember that '''old versions may have security issues. ''' You should also check out [https://www.mozilla.org/en-US/firefox/organizations/ Firefox; Extended Support Release] {web link} [https://www.mozilla.org/en-US/firefox/organizations/notes/ ESR Notes] [https://www.mozilla.org/en-US/firefox/organizations/system-requirements/ System Requirements]

Thanks for your help, i had to downgrade to 5.7 and everything is working correct. Hopefully the next update after 5.8 will work ok. Thanks again

Glad to help. Safe Surfing.

See also: *https://forum.kaspersky.com/index.php?/topic/386238-your-connection-is-not-secure-error-code-sec_error_unknown_issuer-firefox-problem-with-kapersky-kavkiskts-2018/&page=2&tab=comments#comment-2773868